A phase-locked loop locks both the phase and the frequency of a controlled oscillator with those of a reference oscillation. For some applications, in particular coherent detection and selective filtering with cancellation of noise, it is necessary to make the pass band of the closed loop as narrow as possible. This is detrimental to the natural capture (pull-in) properties of the loop which, on the contrary, require the pass band of the closed loop to be as wide as possible.
A known way of meeting these contradictory requirements consists in having a low-pass filter with a switchable structure depending on the instantaneous phase difference between the reference oscillation and that supplied by the oscillator. Due to this switchable structure the low-pass filter has two distinct transfer characteristics, one giving a narrow band to the closed loop after capture and the other shortening the capture time of the loop.